The present invention relates to a scanning apparatus which utilizes a laser, and, more particularly to a variable density scanning apparatus which is capable of changing printing density for use in a laser printer unit.
A conventional scanning apparatus which utilizes a laser beam has a polygonal mirror and a photo-sensitive drum of which rotation speeds are constant so that scanning density for printing is always constant. There has recently been an increasing demand that a graph is to be printed on a paper of different sizes by changing only dimensions of the graph. For this purpose, there have been various proposals according to which the rotation speed of a polygonal is changed and the modulation frequency of a laser is changed in the state that the rotation speed of a photo-sensitive drum is kept constant in order to change the density of printing dots (see JP-A-55-119785, for example). By such an arrangement as described, it is certainly possible to change the intervals (density) of printing dots both in the main scanning direction and the sub scanning directions. However, the spot size of a laser for printing one picture element remains still unchanged in this method. The spot size can be substantially changed in the main scanning direction because the modulation frequency of a laser is changed, but the spot size of a laser in the sub scanning direction does not change at all.
FIGS. 2A and 2B show inconveniences which occur when a spot diameter is not changed even if scanning density is changed. FIG. 2A shows a state when the spot size is optimum under a certain interval of scanning density, and FIGS. 2B and 2C show states when the interval is changed from that of FIG. 2A. A state when the spacing is expanded and a state when the spacing is reduced are shown in FIG. 2B and FIG. 2C, respectively. FIG. 2B shows that a gap occurs, and FIG. 2C shows that unnecessary portions are also blacked out. Picture quality is changed and lowered in these cases. Therefore, when scanning interval (density) is changed, it is necessary that the spot size of a beam to be stopped is also changed occasionally to keep an optimum diameter.